The art of freeze drying has provided an efficient technique for preserving products for human and animal medicinal use. Preservation of plasma, hormones, viral vaccines, enzymes and tissue are but a few of the applications of modern freeze dryers. The process of freeze drying is similar to vacuum distillation, except that the material to be dried must be solidly frozen before it is subjected to a high vacuum and controlled heat input. Four conditions must be obtained to accomplish proper freeze drying. Briefly, the product to be dehydrated must be solidly frozen, secondly, a heat source must be employed to provide the heat of sublimation necessary to drive the water content of the material directly from its solid state to the vapor state, thirdly, a condensing surface is required, and finally, the system must be provided with a vacuum. The end product of this dehydration process is virtually identical to the original material, minus its water content.
In the past, the primary concern of prior art freeze dryer designs was to accommodate heat sensitive materials without particular concern for the corrosive properties of the materials to be dried. Use of freeze dryers in recent years has expanded to include all kinds of materials including those with a relatively high acid or base content. Users of conventional freeze dryers have discovered that the walls, and particularly, the bottom corners of stainless steel condensers, are susceptible to significant corrosion as acidic and basic vapors are condensed there during the freeze drying process. Protective coating such as that denoted under the trademark Teflon .RTM. have been tried, but were found to peel or to be too porous.
Many users have attempted to reduce the corrosion caused by caustic vapors by employing techniques designed to accelerate the defrosting process. However, even prompt removal of the ice formed during dehydration does not completely prevent deterioration. Corrosion begins shortly after the defrosting cycle takes place. The refrigerated fluid circulating through the condensing coils during operation is replaced with a hot gas for defrosting, causing the layer of ice at the interface of the condenser to melt almost immediately. The corrosive effect of the liquid acid or base existing at the interface continues until all of the ice is removed and the condenser fully cleaned and dried.
In addition to problems of corrosion, the stainless steel condensers used in certain prior art structures must be fully cleaned and drained after defrosting. A subsequent user who begins a freeze drying run without completely draining the condenser will most certainly contaminate any oil sealed vacuum pump almost immediately. Large amounts of corrosive materials will permanently damage a pump and most probably necessitate its replacement. Accidents of this nature are not uncommon where a single freeze dryer is used by several departments in the same laboratory. Even assuming proper care is used to clean prior art condensers, subsequent freeze drying runs are delayed until the thawing, cleaning and draining process is completed.